1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof.
A semiconductor device in this specification refers to any device that can function by utilizing semiconductor characteristics. An electro-optical device, a semiconductor circuit, an electronic appliance, and the like are all semiconductor devices.
2. Description of the Related Art
A technique for forming a transistor by using a semiconductor film formed over a substrate having an insulating surface has attracted attention. The transistor is applied to a wide range of semiconductor devices such as an integrated circuit and a display device. A silicon film is known as a semiconductor film applicable to the transistor.
As the silicon film used as a semiconductor film of a transistor, either an amorphous silicon film or a polycrystalline silicon film is used depending on the purpose. For example, in the case of a transistor included in a large-sized display device, it is preferred to use an amorphous silicon film, which can be formed using the established technique for forming a film on a large-sized substrate. On the other hand, in the case of a transistor included in a high-performance display device where driver circuits are formed over the same substrate, it is preferred to use a polycrystalline silicon film, which can form a transistor having a high field-effect mobility. As a method for forming a polycrystalline silicon film, high-temperature heat treatment or laser light treatment which is performed on an amorphous silicon film has been known.
Further, in recent years, an oxide semiconductor film has attracted attention. For example, a transistor which includes an oxide semiconductor film containing indium, gallium, and zinc and having a carrier density lower than 1018/cm3 is disclosed (see Patent Document 1).
An oxide semiconductor film can be formed by a sputtering method and therefore can be applied to a transistor in a large-sized display device. Moreover, a transistor including an oxide semiconductor film has a high field-effect mobility; therefore, a high-performance display device where driver circuits are formed over the same substrate can be obtained. In addition, there is an advantage that capital investment can be reduced because part of production equipment for a transistor including an amorphous silicon film can be retrofitted and utilized.
A transistor including an oxide semiconductor film is known to have extremely low leakage current in an off state (also referred to as off-state current). For example, a CPU with low-power consumption utilizing the low leakage current of the transistor including an oxide semiconductor film is disclosed (see Patent Document 2).